IMPITRITIES OF PHOSPHATE ROCK. 



85 



in the log washer. In Tennessee, where a very considerable part 

 of the brown rock phosphate is in the form of minnte pebbles, 

 settling tanks for saving the small pebble have been introduced in 

 late years, b^rom thq log washer the fine material passes throiigli 

 these tanks^ and much phosphate that was originally thrown into 

 the dump is thus cleaned and saved, and in fact some of the aban- 

 doned dumps in Tennessee are now being rewashed by this method. 

 However, notwithstanding these improvements in mining meth- 

 ods a very considerable amount of phosphoric acid in the form of 

 very fine pebble and so'ft phosphate still finds its way into the waste 

 dump.' It is true that a large part of the phosphate that is thus 

 lost is too high in iron and aluminum to be used in the manufacture 

 of fertilizers under existing processes. It is, however, none the 

 less desirable that the phosphoric acid be saved. To this end it is 

 to be hoped that devices for recovering the very fine pebble phos- 

 phate may be further perfected, and that new process of manufac- 

 ture may be developed in which the grade of rock that can be used 

 is not so strictly limited. 



Fig. 21. — Sample of phosphate illustrating the formation of phosphate by 

 the replacement process. The rock was clearly originally limestone of the 

 Vicksburg formation, the form of the shells being well preserved. The car- 

 bonate has been replaced by phosphate, and the rock as shown by analysis is 

 now a high grade phosphate. Natural size. Specimen contributed by H. R. 

 Keyster, of the Dunnellon Phosphate Company, Rockwell, Florida. 



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